10 



CIRCULAR 7 6 5, U. S. DEPARTMENT OF AGRICULTURE 



Table 3. — Effect of 0.37 to 1.0 p. p. m. of ozone in apple-storage rooms 

 of a commercial cold-storage plant on the numoer of colonies per 

 plate exposed for 30 minutes 





Colonies per plate exposed in — 



Kind of colony 



Room con- 

 taining 0.7- 



1.0 p. p. m. 



of ozone 

 (continuous) 



Room con- 

 taining 0.37 

 p. p. m. of 



ozone 

 (24 hours) 



Xonozonized 

 room 



Penicillium spp. and other fungi ' 



Bacteria 



Number 

 35.0 

 . 3 



Number 

 30. 4 



1. 



X umber 



43. 5 

 .8 



1 The majority of the colonies were Penicillium (34.5. 29.7, and 43.0, respec- 

 tively). 



Further evidence of the ineffectiveness of ozone as a fmnigant was 

 reported by Smock and Watson (££), who found that 0.6 p. p. m. 

 killed spores of Penicillium cjcpansum and Sclerotinia fructicola 

 (Wint.) Rehm equally well when they were wet or dry if they were 

 not in clumps, but that if the spores were protected by moist surfaces 

 of apple flesh or other organic protectants the ozone may have no effect 

 on germination. Hartman (15) likewise stated that ozone in a con- 

 centration of 0.5 to 1.0 p. p. m. was decidedly inhibitory toward nearly 

 all forms of micro-organisms, but that it was not germicidal until 13 

 to 14 mg. per liter (6,500 p. p. m.) was reached. He stressed the need 

 for differentiating an inhibitory from a germicidal concentration. 

 Elford and Van den Ende (4) found that ozone in the very dilute con- 

 centration of 0.04 p. p. m. was able to inactivate certain bacteria when 

 they were present as unprotected singleton aerosol particles, but that 

 when such bacteria were covered by a protective coating, as in a spray 

 (sneeze particles), this concentration was without effect. 



These results substantiate the earlier observation that ozone in very 

 dilute concentrations has an inhibitory or even lethal effect on micro- 

 organisms if they occur singly and unprotected. However, if they 

 occur in clumps or are protected by surrounding films, the ozone is 

 relatively ineffective. 



Spore Load Introduced in Air of Empty Storage Room 



At the end of the storage season, after the apples had been removed 

 from the two test rooms, a series of experiments was conducted to de- 

 termine the effectiveness of high concentrations of ozone in destroying 

 fungus spores in empty storage rooms. To insure a heavy spore load. 

 spores of Penicillivtm expcmsum were introduced into the room at the 

 beginning of each experiment. Petri dishes containing sterile nutrient 

 agar were exposed for 80 minutes before the introduction of ozone and 

 at intervals during the ozonization as a means of determining its 

 effectiveness in killing spores. Air movement was moderate during the 

 experiment. 



The effects of intermittent high concentrations of ozone are illus- 

 trated in figure 4. Large percentages of the spores were killed after 



